I am actually a little bit relieved that I had the M&B series on CD first, before playing on steam, or I would most likely have over 500-1000 hours spread across the 3 games. I absolutely love this series!
Warband is easily the best of the 3 available, with superior mod support, however, the orignal M&B is still very good. If you are able to choose, definitely go with Warhand, which is basically a more fleshed-out version of the original game. While playing the original, I did notice that some of my favorite mods did not get ported over to Warband, so make sure you check out the mod community for M&B to see if anything there interests you.
Much of the aesthetics, controls, and gameplay mirror Warband, so here is my review
There usually is only a few dollars difference between M&B and Warband whenever they both go on sale, so be sure to pick up Warband if possible (or both, if you like collecting).
Despite the more superior Warband being available, this is still a great game, and you should try it out if you are interested in sampling the series.
DJSF @DJSF's Rogue Reviews